Same as CFs only a better design, not being folded they do a better job at getting the light where it's needed using reflectors. Other than that, they are too expensive at the moment in the US, you always pay for novilty, but in time they will be cheap and a bigger selection of bulbs will be available. In Italy I noticed these for sale in hardware stores as shop lights, so you can imagine how cheap they will become in time. I'm sure they'll replace most of the T8 lighting over time, just like T8 replaced much of the T12 lighting fixtures.
T5's are the bulbs that run coolest of all lights that we use in this hobby. The price is not that much higher than CF's.
I tried to use 6 CF's over my 180 gal. tank but the heat was a problem and that was only 330 watts of light.
My only experience with T5's was when I worked on a 240 gals. tank that had sixteen 54 watt T5's. The metalic hoods containing the lamps rest on the glass brackets. There is a lid on top of them. Even with that lid the heat was barely noticeable - and only if you touched the metalic hoods.
That tank was 2 ft. deep and the plants that grew close to the bottom stayed short or grew horizontally which I take to be a sign of enough light.
I was looking at Oliver's site the other day and somewhere there he says that he would prefer T5's instead of HQI. He gave no explanation why.
This is interesting as I'm just beginning to research setting up a large (120-180g) tank. If T-5 can give satisfactory light levels over a 24" tall tank then it definitely opens up more options for me.
The bulbs are similar cost but the fixtures... for me to get are much more expensive then AHsupply fixtures. The lights are good. I still prefer CF. Maybe its just because i am used to them. You wont notice much of a difference anyhow.... a t5 is a CF that is not bent.